Acrylic resins generally have excellent characteristics such as high transparency, high weather resistance, high hardness, ease of processing and the like, and are used for wide applications such as automotive parts, building materials, electrical/electronic products, furniture, vehicles, ornaments, and the like. However, acrylic resins have high flammability, easily ignite and flare up, and thus lead to rapid spread of flame and cause considerable damage. Accordingly, their uses have been limited. Acrylic resins also have a disadvantage of emitting toxic monomers during combustion, and imparting flame retardancy to acrylic resins is desired.
As methods for imparting flame retardancy to acrylic resins, methods including adding a halogen compound or a phosphate compound (PTL 1 and PTL 2) are known. Additionally, a method for imparting flame retardancy in which a halogen compound and a phosphorus compound are added to acrylic resins has been reported, such as a method including using an acrylic resin in combination with a brominated epoxy resin and a phosphorus compound (PTL 3) and a method including using a halogenated phosphate ester (PTL 4). Meanwhile, it is known that even a small amount of halogen makes an effect when an acrylic resin is crosslinked with a brominated acrylic acid compound (PTL 5).
In recent years, techniques for imparting flame retardancy without using a halogen-based flame retardant have been demanded from the viewpoint of environmental problems such as toxic gas emission during combustion. In addition to the aforementioned phosphate compound, a method including combination of red phosphorus with a nitrogen-containing phosphorus compound (PTL 6) has been reported. Additionally, a method for obtaining a flame-retardant acrylic resin by copolymerizing an acrylate monomer and a phosphorus-containing monomer (PTL 7) has been reported. However, in the range of the conventional art, issues are remaining such as insufficient flame retardancy and impairment of transparency, which is a feature of acrylic resins. The only technique that can satisfy both transparency and high flame retardancy was a technique for copolymerizing (an acrylic monomer and) a phosphorus-containing monomer. In such a technique for flame retardancy by copolymerization, it is not easy to adjust the amount of the flame retardant component to be added for achieving desired flame retardant performance. Moreover, for the development of a resin composition with a purpose of improving the physical properties by use of other components to be added, adjustment of the amount of the flame retardant component to be added is required, and a technique for imparting flame retardancy with an addition-type flame retardant has been demanded. In other words, no method has been suggested so far for obtaining a flame retardant acrylic resin having an excellent balance among mechanical strength, surface hardness, flame retardancy, transparency and the like by adding a non-halogen flame retardant not by copolymerization but by melt kneading.
Meanwhile, a method for imparting high flame retardancy by adding an organophosphorus compound to a styrene resin (PTL 8) has been reported, but there was no mention of a technique to impart high flame retardancy while retaining transparency of the acrylic resin.